1. Field of Invention
The present invention relates generally to ship hullforms and, more particularly, to ship designs with both improved seakeeping and improved resistance characteristics.
2. Brief Description of Related Art
In recent years, there has been renewed interest in the multihull ship concept. Most early multihull ship designs were based on the twin-hull catamaran concept wherein two conventional displacement or planing hulls were connected by an above-the-water cross structure. Such ship designs characteristically possess small cross sectional areas in the horizontal plane of the water surface (small waterplane area) when compared to the maximum horizontal cross section of their above-the-water hulls or to the waterplane area of a typical monohull. The principal advantage of small waterplane are ships is their improved seakeeping characteristics in high sea states.
Improved seakeeping (dynamic stability) has been the basis of interest in small Waterplane Area Twin Hull (SWATH ship arrangements. SWATH ships provide a platform relatively isolated from ocean surface disturbances. Buoyancy is provided by submarine like twin-hulls located well below the water surface, which, like submarines, escape the buffeting of wind and waves. The lower hulls are connected by means of surface-piercing struts to the cross structure located well above the water surface. The struts, which pierce the water surface (surface-piercing struts), present a much smaller waterplane area to dynamic wave action than do conventional monohull or catamaran ship hulls which displace and ride on the water surface.
However, the best SWATH ship arrangements require large amounts of propulsive power, when compared with monohulls, because of the large wetted surface area, and consequently high frictional resistance, of the twin-hulls. In addition, multihull ships have an additional component of resistance, interaction resistance between the multiple hulls (also referred to as interference resistance), not experienced by monohull ships. As a result, there has been a continuous search to relieve the large resistance penalty which accrues from the increased wetted surface area and multiple hull interaction of multihull ship compared to monohull ships of the same displacement.
The forces experienced by ships moving in calm water consist primarily of the drag forces attributed to viscous fluid friction (viscous or frictional resistance) and to the production of waves (wavemaking resistance). Frictional resistance is confined to a thin layer adjacent to the ship's surface and is generally dependent upon the wetted surface area of the hullform. Frictional resistance is normally estimated using well known empirical frictional extrapolator methods. Wavemaking resistance of multihull ships is composed of components produced by the wavemaking characteristics of each hull and by the wavemaking interactions of the various hull elements. The wavemaking resistance of thin or slender ships can be analyzed by Michell's thin ship theory. Experimental studies have shown very good agreement between theoretically and experimentally determined wavemaking resistance of slender hullforms such as SWATH ships and other slender hulled multihull ships.
The O'Neill Hullform (OHF), designed for the U.S. Navy for superior roll stability and damage protection of the inner hull against antiship missiles, is a promising example of a trimaran arrangement of hulls. The OHF features a large submerged center body and surface-piercing struts connected to the upper hull. Experimental and analytical studies conducted by the Navy have demonstrated the advantage, in terms of total resistance, of the OHF at speeds above 21 knots when compared to a typical 4000 ton SWATH configuration.
The O'Neill Hullform (OHF) has a large, submerged center hull with a connecting surface-piercing strut and a pair of identical surface-piercing outer strut-like hulls placed symmetrically to the center hull. The center-body-plus-strut provides approximately 80 percent of the ship buoyancy. The outer hulls are shorter in length and smaller in maximum width than the center body.
The wavemaking resistance of multihull ships has three main components: the center hull (body-plus strut for the OHF) component: the outer hull component and the interaction components between the outer hulls and the center hull. In addition, the center hull component of the OHF is composed of components due to the lower body, the surface-piercing strut, and the interaction between the center strut and body due to the existence of a sharp discontinuity at the strut-body intersection.
The inventor has determined that, depending upon the hull geometry and relative positioning among multiple hull elements, the interaction wavemaking resistance component can be either detrimental or favorable to the total wavemaking resistance. Large resistance improvements, therefore, may be possible if proper alterations are made to the relative positioning among the multiple hull elements. Furthermore, since a major components of wavemaking resistance of the OHF arise from the center body and center body/center strut interactions, additional resistance improvements may be possible if proper alterations are made to the elements of the center hull (center body and center strut).